Such an electrochemical measuring cell is disclosed in published German patent application 2,627,271. The electrochemical measuring cell disclosed herein includes an ion selective measuring electrode and counter electrode made of silver which are dipped into an electrolyte of sulfuric acid and a silver sulfate as an additive. When detecting hydrochloric acid for example, the chloride ions dissolved in the electrolyte react with the silver ions and form a silver chloride deposit between the measuring electrode and the membrane. This leads to a change or blockage of the diffusion paths during the course of the measuring operation which, in turn, leads to an unwanted falsification of the measuring result. Accordingly, this measuring cell makes the detection possible by means of the additive but the electrode becomes blocked.
In a measuring cell disclosed in U.S. Pat. No. 5,041,204, the gas sample reaches the semipermeable membrane subjected to the ambient whereupon the component to be detected diffuses through the membrane and is dissolved in the electrolyte. The electrochemical reaction takes place at the interface of the electrolyte/measuring electrode surface. The resulting measurement signal is taken off between the measuring electrode and the counter electrode and is a measure for the concentration of the component to be detected. An additive of copper salt is intended to catalyze the oxidation of the substance to be detected (hydrogen cyanide, sulfur dioxide). Although this additive stimulates the reaction, it does not activate the electrode.